Ratio hoop stretch

In stretch blow molding, there are two ratios that multiply together to provide the blow up ratio BUR. In extrusion blow molding, there is only the hoop ratio (that is the blow-up ratio). In stretch blow molding there is the hoop that is multiplied by the axial ratio. Thus, BUR = Hoop ratio x axial ratio. 

Each of fee aforementioned materials also has its own stretch ratios. In order to understand stretch blow molding it is necessary to understand the terms orientation temperature, blow pressure, blow-up ratio, axial ratio, hoop ratio, and stretch ratios. 

For preform design purposes, three relationships are used the axial or longitudinal stretch ratio (SRj), the circumferential or hoop stretch ratio (SR ), and the BUR, which are defined as follows ... 

Note that the ratio of the ratio of the hoop stress (pR/h) to the axial stress (pR/lh) is only 2. From the data in this question the hoop stress will be 8.12 MN/m. A plastic cylinder or pipe is an interesting situation in that it is an example of creep under biaxial stresses. The material is being stretched in the hoop direction by a stress of 8.12 MN/m but the strain in this direction is restricted by the perpendicular axial stress of 0.5(8.12) MN/m. Reference to any solid mechanics text will show that this situation is normally dealt with by calculating an equivalent stress, Og. For a cylinder under pressure Og is given by 0.5hoop stress. This would permit the above question to be solved using the method outlined earlier. 

To check if the orientation is correct, a dog bone can be cut from the stretch blow-molded PET container and a tensile test conducted on an Instron or similar machine. A dog bone shape is cut in the hoop direction and one is cut in the axial direction. PET has a base strength of approximately 46 MPa (6700 psi). If the hoop ratio is 5, then in the container in the hoop direction, the tensile strength should be approximately 231 MPa (33,500 psi). If the axial ratio is 2, then the tensile strength in the vertical direction would approximately be 92 MPa (13,400 psi). 

Two ratios involving preform-to-product dimensions are utilized to describe biaxial stretch molding. The first of these is the hoop ratio H ... 

The design of the preform in injection blow molding is critical. The preform should be designed to have a wall thickness in the body of the preform anywhere from approximately 0.035 in. ( 1 mm) to approximately 0.200 in. (5 mm). The preform length is designed to clear the inside length of the bottle in the blow mold by approximately 0.005 in. (0.125 mm). Thus there is minimum stretch in the axial direction of the preform when the bottle is blown. The diameter of the core rod is in all practicality determined by the maximum inside dimension (I-dimension) of the finish of the desired container. In determining the wall thickness of the preform in the main body, it is necessary to know what wall thickness is desired in the final blown article plus the maximum inside diameter of the desired blown article. The ratio of the inside diameter of the blown bottle (Dj) to the inside diameter of the preform (D ) is known as the hoop ratio. 

Recently, other kinds of stretch ratios have been defined for blow molding to better define the limits of the process for different materials, The hoop ratio H is defined as the ratio of the maximum outside dimension (diameter) of the finished molded part to the maximum outside dimension of the parison after emerging from the die (parison diameter) ... 

Elongation stress and hoop stress have been used as main stress components in the tooling draw down ratio investigation because the cable extrudate was mainly undo-elongational stretch beyond the die exit However, shear stress and radial stress plots were also ealeulated and presented in Figures 7 and 8 for the convenienee of further analysis. The reduction on the shear stress was 67 %, and the reduction on radial stress was 63 %. 

In stretch blow moulding process, the preform is stretched in both hoop direction and the axial direction as it is blown into Ae mould. Air blow with stretch rod create the final product by stretching the preform to the mould cavity. Injection stretch blow moulding process has two different blowing stages which are pre-blow and final blow. Stretch rod comes to the end of the bottle cavity with preblow assistance. The time in between stretch rod touches the preform and reaches the end of the cavity is crucial in order to get desired stretch ratio. These two points are named point 0 and point 10. Final blow is applied for the rest of the blowing process right after preblow. 

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